Procedures that have been gaining increasing acceptance and widespread usage for the recovery of gold and/or silver from ores, and the like, are the carbon-in-pulp (CIP), and carbon-in-leach (CIL) processes. These procedures are versatile, and effect efficient recovery of the gold and/or silver from the ore.
In a typical CIP process, milled ore is leached in a series of agitated vessels (typically approximately six vessels each having a retention time of about four hours). In the leach vessels the gold and/or silver is largely dissolved from the pulp. After leaching, the pulp moves to the CIP adsorption system, which typically contains about six vessels each having a retention time of about one hour. The pulp is agitated in each of these vessels, which are open to the atmosphere, and in each vessel the pulp is contacted by activated charcoal particles (i.e. carbon granules) that preferentially adsorb gold and silver from the solution. The inventory of carbon granules is continuously or periodically transferred from one vessel to the next in the opposite direction of the flow of the pulp, with carbon discharged from the first vessel in the series ultimately being passed to a gold and/or silver recovery station, while the pulp discharged from the last vessel in the series is leach residue, which can be disposed of.
Resin-in-pulp processes are similar to carbon-in-pulp processes except that an ion exchange resin is used in place of carbon granules. Such processes have not yet received commercial acceptance for Au/Ag leaching.
Conventional CIL processes are similar to CIP processes except that the dissolution and the adsorption of the gold and silver are practiced essentially simultaneously. In a typical CIL procedure, the ground and thickened ore slurry typically passes to a series of about six agitated leach-adsorption vessels, each having a retention time of about four hours. In the agitated leach-adsorption vessels the carbon and ore flow in countercurrent paths in basically the same manner as in the CIP process, with the loaded carbon passed to a recovery stage and the discharged leach residue is disposed of. As in most cyanidation operations, part of the gold and/or silver is typically dissolved in the grinding circuit and in other preliminary processing steps, such as thickening. Although the proportion of the total metal dissolved in these steps is often substantial, subsequent treatment in a series of leach vessels, or leach-adsorption vessels, is typically practiced in order to obtain more complete gold and/or silver recovery.
It has been known for many years that, under certain limiting conditions, the rate of gold dissolution in a cyanide solution is approximately proportional to the partial pressure of oxygen, and that the rate of dissolution can be significantly increased if generally pure oxygen gas (e.g. gas having an oxygen content of about 99 percent or greater) is used instead of air to effect oxidation during the cyanidation process. However this fact has not been taken advantage of commercially.
According to the present invention, it has been found that the combination of (1) the use of oxygen or oxygen-enriched air and (2) a leach-adsorption system employing activated carbon results in an extremely efficient process for treatment of gold and/or silver ores, or the like.
It has been found that not only does oxygen increase the rate of dissolution of gold and/or silver, but that the overall efficiency of processes employing carbon adsorption in gold and/or silver recovery is significantly increased by the use of a gas containing a significantly higher proportion of oxygen than is found in air.
Although activated carbon is well known to be a catalyst in decomposition of cyanide ion by oxygen, surprisingly, it has been found that the use of oxygen rather than air in CIP or CIL systems does not result in unacceptable cyanide consumption, the cyanide consumption being unexpectedly low.
It has been found that the increased efficiency that results from the practice of the present invention has a number of contributing factors. In CIL and CIP processes, the oxygen increases the dissolution rate, which therefore makes the gold and/or silver more readily available for adsorption by the carbon. Also, since the gas that is introduced as a higher proportion of oxygen than natural air, it will also have a significantly lower proportion of carbon dioxide than normal air. Reduced carbon dioxide also increases carbon adsorption efficiency since carbon dioxide reacts with lime in the cyanide solution to form CaCO.sub.3 (as well as causing other problems), which deposits on the carbon granules.
Practicing the invention one can either get a higher percentage of gold and/or silver extraction, or get the same percentage extraction as in conventional facilities only using much less, and/or smaller, equipment, or a combination of these advantages. For instance in a conventional CIL plant, all of the CIL tanks could be reduced to about one-fifth their normal size if oxygen were utilized instead of air to contact the solution. Further, if oxygen is utilized in a leaching process followed by CIP the large agitated leach tanks can each be reduced to about one-fifth their usual size (with commensurate reduction in the residence time in each).
Compared to conventional CIP processes, according to the invention since the gold would be adsorbed almost as soon as it was leached, the driving force for leaching of the gold would be increased, and the "preg" robbing effects in the case of carbonaceous ores would be minimized. Also the tie-up of gold in the in-process inventory would be significantly decreased.
Compared to conventional CIL processes, the process according to the invention would reduce the agitated tank size by a factor of about five or more, reduce the carbon and gold loss due to abrasion of the carbon, reduce the tie-up of gold in the in-process inventory, and reduce the carbon inventory.
The process according to the invention also has the potential for optimizing the leach time for differences in the types of ore utilized. For instance for slow leaching ores, a pressurized leach-adsorption system could be utilized to obtain higher oxygen concentration in the solution. For fast leaching ores, oxygen enriched air could be utilized to provide only a moderate increase in leach rate since little is gained by reducing the leach time below the time required for carbon adsorption (about 4-6 hours). In any event, the practice of the process according to the invention, and the utilization of the apparatus according to the present invention, is extremely advantageous.
It is the primary object of the present invention to provide for the increased efficiency of the recovery of gold and/or silver from ores or the like. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.